OK, a picture is worth 1000 words. I just grabbed a shot of my grandson who REALLY, REALLY, REALLY needs a haircut. Here's the original, trimmed down to fit better here.
I decided to crop it to an 8x10 ratio.
Notice that this is without the perspective box selected. If I drag a corner, it keeps the same aspect ratio, just includes more or less of the image.
Now I did it again with the perspective box ticked:
Nothing else changed, except I dragged the top down and the bottom up. Doesn't look like an 8x10, right? But here's what you get when you invoke the crop by double clicking inside the selected area:
Elongated face! The reason is that with the perspective box selected, the cropping tool will take whatever's selected and make it fit in an 8x10 aspect ratio. It will stretch or shrink it as necessary to make that happen.
I'm guessing this is what's going on for you.
The perspective control is a very powerful tool. You can use it to make lines that are not parallel, parallel (couldn't think of a better way to say that. Oh wait: a picture is worth...)
Drag the cropping rectangle (OK, not a rectangle any more, a quadrilateral) until the sides are parallel with the things you want straight up and down or sideways
Perspective cropping in Photoshop. That's how it works.